Rosetta and Philae Capture First Detailed Magnetic Measurements of a Comet Nucleus

By Ken Kremer, on April 23rd, 2015

Comet 67P/C-G’s activity! This stunning montage of 18 images from Rosetta’s navcam camera shows off the comet’s activity from many different angles. It covers the time period between 31 January (top left) and 25 March (bottom right), 2015, when the spacecraft was at distances of about 30 to 100 km from the comet. The final frame is from 25 March. At the same time, Comet 67P/Churyumov-Gerasimenko was at distances between 363 million and 300 million km from the Sun. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Europe’s history-making cometary explorers—Rosetta and Philae—have captured the “first ever detailed measurements of the magnetic properties of a comet nucleus” in our Solar System and found that their target comet 67P/Churyumov-Gerasimenko is “not magnetized,” according to the research team leading the project.

Mission scientists reported the magnetic measurement results in a new paper published in the journal Science and presented at the European Geosciences Union General Assembly in Vienna, Austria, on April 14, 2015.

The tiny, dual-lobed body is shaped like a rubber ducky and measures only about 4 kilometers (2.5 mile) wide.

Together, Rosetta and Philae are the Earth’s first spacecraft in history to successfully and safely orbit and land on a comet and conduct unprecedented up-close science explorations otherwise not possible.

“Studying the properties of a comet can provide clues to the role that magnetic fields played in the formation of Solar System bodies almost 4.6 billion years ago,” say ESA officials.

One of the looming questions that the team seeks to answer is whether magnetic forces played a significant role in the accumulation of planetary building blocks at a time when the Solar System was in its infancy and “nothing more than a swirling disc of gas and dust.”

Indeed, the magnetic measurements are only made possible due to the never-before-accomplished close proximity of ESA’s Rosetta orbiter to Comet 67P and the Philae lander’s touchdown on the pockmarked surface that took place later on Nov. 12, 2014, four months after orbital arrival last year.

Previous comet missions from the world’s space agencies involved fast flybys from far greater distances of the comet’s nucleus where “detecting the magnetic field of comets has proven difficult.”

What’s even more fortuitously fascinating is that the totally unplanned multiple bounces across the comet following Philae’s initial landing was actually a boon to the researchers who had designed and built the particular science instruments involved.

Philae’s magnetic field measuring instrument is the Rosetta Lander Magnetometer and Plasma Monitor (ROMAP), while Rosetta carries a magnetometer as part of the Rosetta Plasma Consortium suite of sensors (RPC-MAG).

After Philae was deployed from Rosetta on Nov. 12, 2014, it touched down with high precision as planned at the intended Agilkia landing site. But then it unexpectedly bounced three times, making for four individual points of contact.

Reconstructing Philae’s trajectory. Magnetic field data from ROMAP on Philae, combined with information from the CONSERT experiment that provided an estimate of the final landing region, timing information, images from Rosetta’s OSIRIS camera, assumptions about the gravity of the comet, and measurements of its shape, have been used to reconstruct the trajectory of the lander during its descent and subsequent landings on and bounces over the surface of Comet 67P/Churyumov-Gerasimenko on 12 November 2014. The times are as recorded by the spacecraft; the confirmation signals arrived on Earth 28 minutes later. Credit: ESA/Data: Auster et al. (2015)/Comet image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The ROMAP instrument, therefore, was able to capture a bevy of extra magnetic measurements during the multiple unplanned descents and ascents at varying altitudes, which greatly increased the science data return.

“The unplanned flight across the surface actually meant we could collect precise magnetic field measurements with Philae at the four points we made contact with, and at a range of heights above the surface,” says Hans-Ulrich Auster, co-principal investigator of ROMAP and lead author of the results published in Science and presented at the EGU meeting, in a statement.

In fact, at one point Philae made a grazing collision with a cliff. After the initial landing, Philae bounced back up and began rotating much faster than it did during the initial descent to the surface.

“It collided with a cliff 45 minutes later, then tumbled, flying above the surface for more than an hour longer, before bouncing once again and coming to a stop a few metres away, a few minutes later,” the team concluded.

“The position of the first touchdown point at Agilkia is very well determined from direct images, but the locations of the possible cliff collision depends on the ballistic model used, while the general location marked for the subsequent second and third touchdowns at Abydos come from the CONSERT measurements. Thus, these latter positions represent preliminary and approximate locations only.”

Although puzzled at first and thinking the lander set down only once, as planned, the lander team, led by the German Aerospace Center (DLR), soon determined that Philae had bounced three times.

“By sensing periodic variations in the measured external magnetic field and motions in its boom arm, ROMAP was able to detect the touchdown events and determine the orientation of Philae over the following hours. Combined with information from the CONSERT experiment that provided an estimate of the final landing site location, timing information, images from Rosetta’s OSIRIS camera, assumptions about the gravity of the comet, and measurements of its shape, it was possible to determine Philae’s trajectory,” according to the ROMAP team.

“Philae came into contact with the comet’s surface four times in fact – including a grazing collision with a surface feature that sent it tumbling towards the final touchdown point at Abydos.”

“This complex trajectory turned out to be scientifically beneficial to the ROMAP team,” noted ESA.

ROMAP measured a magnetic field during these sequences, but found that its strength did not depend on the height or location of Philae above the surface. This is not consistent with the nucleus itself being responsible for that field.

“If the surface was magnetised, we would have expected to see a clear increase in the magnetic field readings as we got closer and closer to the surface,” explains Auster.

“But this was not the case at any of the locations we visited, so we conclude that Comet 67P/Churyumov-Gerasimenko is a remarkably non-magnetic object.”

The data from the ROMAP and RPC-MAG instrument showed that “the magnetic field that was measured was consistent with an external one, namely the influence of the solar wind interplanetary magnetic field near the comet nucleus. This conclusion is confirmed by the fact that variations in the field that were measured by Philae closely agree with those seen at the same time by Rosetta.”

“During Philae’s landing, Rosetta was about 17 km above the surface, and we could provide complementary magnetic field readings that rule out any local magnetic anomalies in the comet’s surface materials,” says Karl-Heinz Glassmeier, principal investigator of RPC-MAG on board the orbiter and a co-author of the Science paper, in a statement.

No large chunks of magnetization material were found on the comet’s nucleus in readings from ROMAP during its flight over the surface.

“If any material is magnetised, it must be on a scale of less than one metre, below the spatial resolution of our measurements. And if Comet 67P/Churyumov-Gerasimenko is representative of all cometary nuclei, then we suggest that magnetic forces are unlikely to have played a role in the accumulation of planetary building blocks greater than one metre in size,” concludes Auster.

Overall, the data show that the comet has an upper magnetic field magnitude of less than 2 nT at the cometary surface at multiple locations, with a specific magnetic moment of < 3.1 x 10–5 Am2/kg, less than known values for lunar material and meteorites measured on Earth, reports the team.

“It’s great to see the complementary nature of Rosetta and Philae’s measurements, working together to answer this simple, but important ‘yes-no’ question as to whether the comet is magnetised,” concludes Matt Taylor, ESA’s Rosetta project scientist.

Rosetta is currently escorting comet 67P around the Sun. The comet’s activity and outpouring of gas and dust have picked up dramatically as the pair veer in ever closer toward the Sun and feel the impact of its warming.

The increased activity recently caused significant navigation issues with the comet and confused the star trackers, which maintain the probe’s orientation and communication with Earth.

As a result, the team is now “re-thinking” the strategy on how they will approach the spewing ice ball during future flybys, detailed here.

“We are re-thinking how we go about ‘orbiting’ the comet,” Taylor told AmericaSpace in the wake of the significant navigation issues experienced by the spacecraft after it flew within 14 kilometers (8.7 miles) of the surface of comet 67P on March 30.

Comet 67P and Rosetta will reach perihelion on Aug. 13, 2015, at a distance of 186 million km from the Sun, between the orbits of Earth and Mars.

That’s only four months from now, and the excitement is non-stop as the comet belches ever more gas and dust particles!

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Parting post flyby whole body view of ducky shaped comet 67P. Four image mosaic of Comet 67P/Churyumov-Gerasimenko comprising images taken on 14 February 2015 at 19:42 GMT from a distance of 31.6 km from the comet center. The image scale is 2.7 m/pixel. Rosetta’s parting shot following the close flyby features the comet’s small lobe at the top of the image, with the larger lobe in the lower portion of the image set. Credits: ESA/Rosetta/NAVCAM/Ken Kremer/kenkremer.com/Marco Di Lorenzo